This invention in general relates to three-phase power devices (such as three-phase motors) and, more particularly, to an improved procedure for measuring the currents in each phase via a single current sensor.
A three-phase motor (such as a permanent magnet synchronous motor and induction motor) is used in automotive applications such as power steering systems. It is known to control the phase windings in a three-phase motor using pulse width modulated signals. The pulse width modulated signals are applied to an inverter or a series of switching devices that connect the phase windings of the motor to either a positive or negative terminal of the vehicle battery.
To adequately control the motor, it is necessary to measure the current flowing through each phase winding. The current measured in each phase is provided to a controller that generates the pulse width modulated signals. To measure currents, it is known to use a resistor in series with each phase winding. The voltage drop across each resistor is measured to determine the current flowing for each phase. This type of system has the disadvantage of requiring three current sensors, one for each of the three phase windings.
It is also known to reduce at least one of the current sensors and calculate the current through the phase without a sensor based on the measurements of the other two phases. Since the sum of all currents flowing through all phases of the motor must be zero, knowing the current flowing through two phases allows a determination of the current flowing through the phase without a sensor.
It has also been proposed to use a single resistor to perform the function of measuring the current through each phase of the three-phase motor. The single resistor is located on the DC link between the DC power supply and the inverter or series of switching devices. Depending on the states of the switches and period of operation, the currents through the phases may be measured or calculated.
A system using a single DC link current sensor is described in European Patent Application EP 1 083 640 A2. There, the system proposes to shift certain pulse width modulated signals for measuring currents during certain times.
Recent advances in digital signal processors (DSPs) have permitted the use of more advanced pulse width modulation schemes such as space vector pulse width modulation (hereinafter xe2x80x9cSVPWMxe2x80x9d). One significant advantage of using SVPWM is that it can provide 15% more output voltage compared to conventionally known sinusoidal pulse width modulation schemes. The drawback, however, is that SVPWM requires more complex schemes to measure full phase currents via a single DC link current sensor. For instance, there are situations where the full phase currents cannot be sampled during SVPWM operation. One situation is when the amplitude of the voltage space vector is very small. Another situation is when the voltage space vector falls on one of the active vectors. During these situations, the system may be unable to reliably measure or calculate the current through all three phases of the windings in the motor.
Further improvements are needed to known systems to fully utilize SVPWM in three-phase motor applications. In particular, a need exists for improved procedures and schemes to shift pulse width modulated signals to account for a variety of cases where the reliability of the measurement may be in question. Moreover, a continuing need exists for improving the accuracy of the current measurements for each phase. It is, therefore, desirable to provide an improved procedure for reliably and more accurately measure the currents flowing in each phase of a three-phase device to overcome most, if not all, of the preceding problems.